1. Field of the Invention
The present invention relates to an image forming method and image forming system for forming an image.
2. Description of the Related Art
As an image processing apparatus for printing and outputting an image, an inkjet printer which uses a plurality of inks as color materials is widely known. A printer of electrophotographic scheme or the like is also known, which uses toners as color materials. These image forming apparatuses express various colors by subtractive color mixture using three color materials of cyan (C), magenta (M), and yellow (Y) or four color materials including black (K), thereby printing.
In this kind of printing (image formation), when printing is performed based on signal values that decide the color material amounts of C, M, Y, and the like, faithful reproduction of the colors intended by the signal values fails in not a few cases. For example, if the size of dots formed by the color materials on a printing medium such as a paper sheet slightly varies, the colors of a printed image formed from dot aggregations are observed with a subtle color shift. This occurs when, for example, the amount (volume) of discharged ink droplets slightly changes due to the individual difference of the printhead that discharges inks, or the size of latent dots formed on the photosensitive member of an image forming apparatus of electrophotographic scheme slightly changes. The small change in the dot size can also occur depending on the relationship between the type of a printing medium in use and the properties of color materials such as inks or toners. The size of dots to be formed can also change because of aging of the image forming apparatuses.
The above-described phenomenon in which the colors of an actual printed image are expressed with a shift from colors (position coordinates) intended by the color material signals in the color space can take place in many image forming apparatuses. In this specification, this phenomenon will be referred to as a “color shift”.
So-called calibration is conventionally known as a technique for coping with color shift. For example, after a target printer prints a patch, the color material amounts are adjusted by changing or generating tables for color conversion and γ correction based on the colorimetry result, thereby suppressing the color shift. In addition, the volume of ink droplets discharged from each printhead is measured, and image processing is changed based on it, thereby adjusting the color material amounts.
However, a color shift in, for example, a monochrome image expressed by achromatic colors such as black and gray is relatively hard to adjust. Conventionally, gray is often expressed by superimposing the three basic colors C, M, and Y in almost the same amount especially in a low density portion (Japanese Patent Laid-Open No. 2000-198227). In this case, a subtle variation in the amount of each color material causes an imbalance of the three colors and changes the hue relatively largely. For this reason, color material adjustment itself becomes difficult. Additionally, for the same reason, a slight change in the size of formed dots relatively largely changes the color. Since a color shift of gray means that a chromatic color faintly tints the achromatic color, the color shift is observed conspicuously.
FIG. 12 is a graph showing details of a color conversion lookup table (LUT) described in Japanese Patent Laid-Open No. 2000-198227, which is to be used to print a gray image. The abscissa represents density levels (density values) 0 to 255 expressed by, for example, 8-bit RGB input data for colors on the gray axis of the color space. The ordinate represents the output signal values (0 to 255) of the respective ink colors, that is, ink amounts to express the density values. As shown in FIG. 12, gray is expressed by three colors C, M, and Y from the lower density region to the medium density region. That is, the output values of three color inks shown in FIG. 12 are defined to express gray without any color shift in a predetermined color space. When the input density level roughly exceeds 11, use of black ink (K) starts. At the maximum density level, the output signal value is approximately 128.
FIG. 13 is a graph showing another example of the conventional color conversion LUT. This graph illustrates the ink amounts of the respective colors to express colors on the gray axis of the color space, like FIG. 12. FIG. 13 shows an example of a color conversion LUT when using light cyan (lc) and light magenta (lm) inks which contain colorants such as dyes in low concentration, in addition to cyan (C) and magenta (M) inks.
More specifically, in the recent field of inkjet printers, high-quality images comparable to silver halide photos are demanded. In this case, one of serious obstacles is graininess a printed image gives to an observer. The graininess is a kind of rough appearance an observer perceives when dots formed on a printing medium are so noticeable that he/she can visually recognize them. To reduce the graininess, a plurality of kinds of inks containing colorants in different concentrations are used as inks of the same color to change the print density per droplet as described above.
As shown in FIG. 13, in the low density region, gray is expressed using three color inks lc, lm, and Y. In a process where the density gradually rises from the low density region to the higher density region, dots are formed discretely. Hence, the graininess is reduced by using inks of lower densities. In this example as well, the output values of the three color inks are defined to express gray without any color shift in a predetermined color space. Near the middle density region, the output values of lm and lc are almost maximized. Higher densities can hardly be expressed by combining these inks. On the other hand, in this density region, the surface of the printing medium is filled with many dots. Hence, graininess of single dots is unnoticeable. From near this region, C, M, and K are gradually added to increase the density while reducing the graininess. Simultaneously, the output values of lc, lm, and Y gradually decrease. Finally, the output value of K becomes larger than those of the remaining inks so that black or gray with satisfactory tonality can be expressed.
In the above-described ink amount deciding methods shown in FIGS. 12 and 13, however, color shift adjustment in a gray or black monochrome image is difficult, and the color shift is noticeable, as described above.
Japanese Patent Laid-Open No. 2000-198227 describes a mode to print black characters and the like in which colors on the gray axis are expressed using a black (K) ink in all regions from the low density region to the high density region. If the graininess need not particularly be taken into consideration, gray or black can be expressed using the K ink in all density regions. This makes it possible to suppress a color shift caused by imbalance in expressing, for example, gray by three colors C, M, and Y.
In this case, however, a color shift may occur, which adds an unintended tint to a monochrome image depending on the property of K ink itself or the relationship between K ink and a printing medium.
Japanese Patent Laid-Open No. 2000-198227 has not suggested that the K ink should be used in all density regions for colors except those on the gray axis. In the monochrome photos, an image is sometimes required to be tinted to some extent to express a completely neutral tone (to be referred to as a pure black tone hereinafter), bluish black (to be referred to as a cold black tone hereinafter), or yellowish black (to be referred to as a warm black tone hereinafter).
However, according to Japanese Patent Laid-Open No. 2000-198227, it is impossible to meet the requirement for printing a variety of monochrome images complying with the user tastes. Properly speaking, it is preferable to perform appropriate color conversion processing for all monochrome photo tones (warm black tone, pure black tone, and cold black tone) and print a monochrome image without any color shift. The technique described in Japanese Patent Laid-Open No. 2000-198227, which uses the K ink in all density regions for a pure black tone color, cannot obviously meet the requirement by itself.
For a recent image forming apparatus such as an inkjet printer, a technique of printing by discharging smaller ink droplets has been developed to obtain higher image quality equal to that of a silver halide photo. When a printed image is formed from such relatively small dots, a “color shift” in printing a monochrome image further poses a problem.
More specifically, when ink droplets to be discharged become smaller, a printed image is more seriously affected by a small variation between printheads or a variation in the discharge amount between nozzles of the printhead generated upon production of printheads, or a faint change in the discharge amount depending on the use frequency of the printhead. Each of small dots formed by the small ink droplets original has a small area factor, that is, a small area or ratio of coverage on the surface of a printing medium. Hence, the above-described variation in the discharge amount or the like relatively largely changes the area factor on the entire image, resulting in larger variation in gray.
If the shift amount or direction of the “color shift” changes relatively largely, an “undesirable color change” may occur, which is a phenomenon of the color shift and largely shifts the development. Especially, the “undesirable color change” readily occurs in the process where the inks used dominantly gradually change from the low density region to the higher density region, as described above with reference to FIG. 13.
When using K ink in all density regions for a pure black tone color, yellow ink that is a chromatic color ink is added in a very small amount as a toning component for a warm black tone. For a cold black tone, cyan ink (or light cyan ink) and magenta ink (or light magenta ink) that are chromatic color inks are added in a very small amount as toning components. There can be a technique of enabling color adjustment of a monochrome photo by gradating these colors.
However, when adjusting from the warm black tone to the cold black tone, ink use becomes discontinuous from the pure black tone to both sides. Hence, it may be difficult for a user who has viewed the warm and cold black tone colors to predict the pure black tone color.